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I can't point to BMW direction for using air above the oil as part of the suspension...I suspect that for 98% of average riders, it doesn't really make a difference. However, my quote of Tom Cutter was specific to our BMW forks and he is a racer and knows a bit or two about setting up a bike. The other parts of that discussion by Tom was about the lengths that some people go to measuring the oil volume such that the amount of air in the forks for their setup. So, it is a very real thing...maybe not for the average rider.

As for putting an air valve on top of the fork, I don't think I can find it, but I do recall reading about doing that sometime in the past 15 years. That's all I can remember...it's been discussed before...not here but on the Airhead email list.

I can't point to BMW direction for using air above the oil as part of the suspension...I suspect that for 98% of average riders, it doesn't really make a difference. However, my quote of Tom Cutter was specific to our BMW forks and he is a racer and knows a bit or two about setting up a bike. The other parts of that discussion by Tom was about the lengths that some people go to measuring the oil volume such that the amount of air in the forks for their setup. So, it is a very real thing...maybe not for the average rider.

As for putting an air valve on top of the fork, I don't think I can find it, but I do recall reading about doing that sometime in the past 15 years. That's all I can remember...it's been discussed before...not here but on the Airhead email list.

Actually, I am FOR giving your suggestion a try! You know me I sometimes go "outside the box" on such issues. Many times just because it has never been done, doesn't mean that it can't be done.

However the thread started with two questions: 1) does one have to do anything special to get the air out of the oil when refilling, and 2) can one use more oil to affect the for "shock" action. Just increasing oil as you suggest will not affect the "shock" action, but the spring action which are two distinct functions of the forks.

the shock action can only be modified by changing fork oil viscosity, not quantity.

Using the oil to perform or back up the spring is probably not a good thing, but could be achieved by reducing the amount of air in the forks - by increasing the amount of oil. I am actually open to experiment with this idea. Just because an engineer didn't say it, doesn't mean that it can't be done.

Actually, the OP said "adjust their ride". That can be either the damping side or the spring side, can't it? I'm not sure what the pure definition is of a shock (aka suspension), but I think it involves both damping and springs. The only way to affect the damping is to change the oil and/or change the holes that the oil has to flow through. Springs (coil or air) are the other side of the suspension.

"in the past 15 years"

I know it is not BMW, but my son has a 1983 Yamaha Secca XJ 900. It has an air valve on the left side of the triple tree. It is cross drilled to both fork tubes and sealed with 2 "O" rings on each tube. This is factory. The preasure can be varied to effect the dampening action.
Also along these lines, when i used to go to watch the races at Blackhawk Farms(www.blackhawkfarms.com), there was a Norton being raced on which they were trying an air over oil set up. He had drilled out and tapped the fork tube bolts for air lines and had a preasure gauge mounted to the top tripple tree. He said the set-up would lose some preasure during the course of a race, as it needed better sealing.
Just throwing this into the discussion.
frank

I know it is not BMW, but my son has a 1983 Yamaha Secca XJ 900. It has an air valve on the left side of the triple tree. It is cross drilled to both fork tubes and sealed with 2 "O" rings on each tube. This is factory. The preasure can be varied to effect the dampening action.
Also along these lines, when i used to go to watch the races at Blackhawk Farms(www.blackhawkfarms.com), there was a Norton being raced on which they were trying an air over oil set up. He had drilled out and tapped the fork tube bolts for air lines and had a preasure gauge mounted to the top tripple tree. He said the set-up would lose some preasure during the course of a race, as it needed better sealing.
Just throwing this into the discussion.
frank

those were designed for those methods and purpose (racing with a quick method of adjustment). What about the forks in question on the BMW - designed or usable for that as well?

I think practically, that adjusting the air cap (by changing the oil level) is not really all that critical for our "plush" front suspensions. But I found another series of posts from about 4 years ago by Tom Cutter who suggests that it is something to be considered. He's big into suspensions (he races and sells a specific brand of suspensions and springs that can be customized) so I figure he knows what he's talking about. Tom made reference to this chart by Wibers:

One section is for BMW and the one column to the right that has a cloud over it is the air cap over the oil. Probably a lot more going on in this chart, but it does appear that the amount of oil could be fine tuned for each rider. But in general, the value listed in Haynes or the manuals will cover a wide range of riders and situations.

An easy way to visualize the effect of oil LEVEL in the fork, is to start by visualizing a fork with the springs removed (minimum mechanical spring effect) and NO air (maximum oil level=full, maximum pneumatic spring pneumatic spring rate) . It will not compress.

As you lower the oil level, you create a compressible air pocket. The larger the air pocket, the farther the fork must travel to bring the air pressure up to a given pressure. This air pressure resists fork compression just like the spring resists fork compression, except that as the air space gets smaller the spring rate gets higher ( the additional force required to compress the fork the NEXT inch is higher than the force required to compress the fork the first inch) this is known as a rising rate spring effect.

In a fully assembled fork, with springs and an air space, the two spring rates are additive. The smaller the air space, the quicker the pneumatic spring rate becomes significant. The larger the air space, the further the forks will compress before any significant pneumatic spring effect happens. With a very large air space, pneumatic spring rate could almost be negligible.

Ride report

I did a ride test (OK, twist my arm to get me out on the thing...) and I am very pleased with 235cc of 7.5 fork oil. It rides very nicely...not as harsh as before the change..either the PO had more fluid or thicker fluid, but I like the new ride.

An easy way to visualize the effect of oil LEVEL in the fork, is to start by visualizing a fork with the springs removed (minimum mechanical spring effect) and NO air (maximum oil level=full, maximum pneumatic spring pneumatic spring rate) . It will not compress.

As you lower the oil level, you create a compressible air pocket. The larger the air pocket, the farther the fork must travel to bring the air pressure up to a given pressure. This air pressure resists fork compression just like the spring resists fork compression, except that as the air space gets smaller the spring rate gets higher ( the additional force required to compress the fork the NEXT inch is higher than the force required to compress the fork the first inch) this is known as a rising rate spring effect.

In a fully assembled fork, with springs and an air space, the two spring rates are additive. The smaller the air space, the quicker the pneumatic spring rate becomes significant. The larger the air space, the further the forks will compress before any significant pneumatic spring effect happens. With a very large air space, pneumatic spring rate could almost be negligible.

I think your explanation is spot on. Very well done and accurate.

I would suspect that the amount of air space as designated by BMW's recommendations is probably in the non-significant category. My hesitation (which is contrary to my usual nature!) is to think that BMW didn't engineer these forks to be significantly sprung by air. However, I was reminded by Kurt, who was correct, that my usual position on these types of matters is to give new ideas a try (whether engineered by BMW or not!) and see what works!

I just have a "gut" feeling that that it has been tried and probably in theory works, but they found out that the results weren't satisfactory. Hence the lack of on-going conversation about the concept.

As a matter of fact, I have been working on my forks lately, and may experiment myself. My problem is I want to do it scientifically, but don't have that kind of time.

For me, and probably most casual riders (not racers), the front forks are adequate as long as they are in good condition with good springs. After all, they weren't great shocks/forks when they were new, but they were adequate for most riders.

I really enjoy this back and forth conversations as it really forces (me) to think using other's ideas and perspectives. Just this one thread has taught me a lot! Helps me understand physics principles on a variety of subjects (pneumatic, hydraulic, spring tensions, etc).

I did a ride test (OK, twist my arm to get me out on the thing...) and I am very pleased with 235cc of 7.5 fork oil. It rides very nicely...not as harsh as before the change..either the PO had more fluid or thicker fluid, but I like the new ride.

Thanks to all..and a good spirited discussion too

S

I may get shot for this, but here is what I just did:

After doing a minor rebuild this past weekend, I went with 250cc in each fork of 5w BelRay fork oil. Took the bike out yesterday afternoon, and I was happy with the performance on the roads and interstate (certanly better than before). Though, I am not a fussy rider. My main concern is a smoothly running engine. A plush ride isn't that much on my radar - that's for those guys who have a ton of money who can afford those big bikes that are more like an auto on 2 wheels - a big boat. That's why I usually like running both my front and back shocks/springs in stiff mode. It makes me feel like a "tough" guy!!!

There is a slew of 70s bike models that had "air assist" forks. One of them the Honda CBX, first had it on the 1980 model. As the forks are almost identical to the 79 model, many modify their 79s to "air assist" by using 80 parts (fork caps and valves) I am not sure, whether the additional air pressure puts extra load on the fork seals and makes them leak easier. But when I park my KZ1300 with "air assist" forks for the winter, it will leak after just sitting for while. It does not leak while I ride it all summer. When I release the air pressure before storage, it will not leak either.

Since the pneumatic effect is only significant when the forks are significantly compressed, many manufactures of traditional style forks would use the pneumatic effect to help prevent fork bottoming on forks fitted with relatively soft springs. That way the could get a plush ride through most of the forks travel and have the pneumatic effect stiffen the forks as they approach being bottomed out.

Progressive springs and dual rate springs (a softer main spring with a short stiffer spring in each fork tube such as in the early K bike sport forks) kind of do a similar thing but can allow for more flexible design of suspension qualities.

Since the pneumatic effect is only significant when the forks are significantly compressed, many manufactures of traditional style forks would use the pneumatic effect to help prevent fork bottoming on forks fitted with relatively soft springs. That way the could get a plush ride through most of the forks travel and have the pneumatic effect stiffen the forks as they approach being bottomed out.

Progressive springs and dual rate springs (a softer main spring with a short stiffer spring in each fork tube such as in the early K bike sport forks) kind of do a similar thing but can allow for more flexible design of suspension qualities.

Back in "The Day", it was an experimental practice and even written up in "The News" as a technical item (Don't ask for a footnoted reference). Folks would drill out the top cap and insert a valve stem. Then put in a few pounds of air.........It did exactly as stated above; BUT......there were seal problems and the practice died a lingering death over perhaps a season or two. This is when folks were installing the Reynolds bottoming springs, PVC spacers, and progressive rate springs were just beginning to be developed. It is interesting to note that "hybrid" experimentation and evolution became the standard of today...........God bless......Dennis

Back in "The Day", it was an experimental practice and even written up in "The News" as a technical item (Don't ask for a footnoted reference). Folks would drill out the top cap and insert a valve stem. Then put in a few pounds of air.........It did exactly as stated above; BUT......there were seal problems and the practice died a lingering death over perhaps a season or two. This is when folks were installing the Reynolds bottoming springs, PVC spacers, and progressive rate springs were just beginning to be developed. It is interesting to note that "hybrid" experimentation and evolution became the standard of today...........God bless......Dennis

By adding air pressure to the forks, you are increasing the initial pressure in the air pocket to above atmospheric. Sort of like preloading a spring. That will cause the pneumatic function to be stronger and at an earlier ( less fork compression) point.

have you actually done this? Seen it done?
I ask, because I suspect that the only thing one would "feel" is a fork fluid shower upon the first compression action.

I had caps that wouldn't seal with the currently supplied crush (that don't seem to) washers. I replaced them with Viton o-rings and found quite a difference. Not so much a shower as a modest sheen emerging from the cap area after a ride on a rough road.